The invention relates to the removal and reduction of nitrogen oxides from combustion flue gases, either from coal-fired power-plants, liquid-fuel power generation or natural gas power generation.
Coal, the primary U.S. resource for producing electricity, generates over a third of the nation's electricity. The primary gas emissions are criteria pollutants (e.g., sulfur dioxides [SOx], nitrogen dioxides [NOx], particulate matter, carbon monoxide and others). NOx emissions are chemical oxides that are produced during combustion at high temperatures. The two elements, nitrogen and oxygen typically do not react with each other at normal temperatures, but when high temperature combustion occurs, the elements combine to form toxic oxides.
About two thirds of all SOx and a quarter of all NOx in our atmosphere come from electric power generation, achieved by burning coal and other hydrocarbon fuels. SOx causes acid rain, while NOx creates atmospheric ozone. Secondary emissions, such as mercury, arsenic, selenium, and boron, are indigenous to the type of coal being combusted.
NOx compliance requires use of controls, such as: selective catalytic reduction (SCR), selective noncatalytic reduction (SNCR), and low NOx burners. Both SCR and SNCR cause ammonia slip, which allows un-reacted ammonia to slip past the NOx control device for subsequent removal in the electrostatic precipitator or scrubber. If the water is discharged to an ash pond, this ammonia may oxidize to nitrite and nitrate. The release of these toxins into the atmosphere is considered major contributors to air pollution, and for industries and manufacturers of internal combustion engines, NOx reduction is now a necessity.
In addition to its own toxicity, NOx combines with other substances and creates smog when it is exposed to sunlight. This creates a major health hazard for people that live in affected areas. NOx penetrates into the lung tissue and can cause permanent damage to lung tissue. Children, the elderly, people with asthma and other breathing difficulties, and people that work outside are especially susceptible to the negative health effects of NOx. The way to reduce that health risk is for companies to employ NOx reduction measures.
Nitrogen oxides can also combine with water to produce nitric acid. When released from the atmosphere, it can cause acid rain, and results in significant long term damage to any structure that is repeatedly exposed to it.
In 1997 the Kyoto Protocol classified NOx as a greenhouse gas, and also called for a world wide effort to reduce the amount that was being released into the atmosphere. In the United States, this is regulated by the Environmental Protection Agency (EPA). They have set NOx levels which can legally be released by companies. If the company does not comply with the regulations sanctions and significant fines can be leveled against them.
Many industrial and utility operations are using a selective catalytic reduction (SCR) method for removing nitrogen oxides from the atmosphere. SCR uses an ammonia- or urea-based reductant that combines with the NOx and is absorbed by a catalyst. Carbon dioxide (CO2) is a reaction product when urea is used as the reductant. Once the NOx is absorbed by the catalyst, a chemical reaction takes place to separate the nitrogen and oxygen. The result is that instead of NOx, these operations are now releasing nitrogen, oxygen or water into the atmosphere, all of which are harmless.
The chemical equations for a stoichiometric reaction, using either anhydrous or aqueous ammonia for a selective catalytic reduction (SCR) process is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Some undesirable secondary reactions are:2SO2+O2→2SO3 2NH3+SO3+H2O→(NH4)2SO4 NH3+SO3+H2O→NH3HSO4 
The reaction for urea, when used instead of either anhydrous or aqueous ammonia is:4NO+2(NH2)2CO+O2→4N2+4H2O+2CO2 
The ideal reaction has an optimal temperature range between 630 and 720 K, but can operate from 500 to 720 K with longer residence times. The minimum effective temperature depends on the various fuels, gas constituents, and catalyst geometry. Other possible reductants include hydrogen gas, cyanuric acid and ammonium sulfate.
SCR catalysts are manufactured from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components are usually either oxides of base metals (vanadium, molybdenum and tungsten), zeolites, or various precious metals. Each catalyst component has its own advantages and disadvantages.
Base metal catalysts, such as vanadium and tungsten, lack high thermal durability, but are less expensive and operate very well at the temperature ranges most commonly seen in industrial and utility boiler applications. Thermal durability is particularly important for automotive SCR applications that incorporate the use of a diesel particulate filter with forced regeneration. They also have a high catalyzing potential to oxidize SO2 into SO3, which can be extremely damaging due to its acidic properties.
Zeolite catalysts have the potential to operate at substantially higher temperature than base metal catalysts; they can withstand prolonged operation at temperatures of 900 K and transient conditions of up to 1120 K. Zeolites also have a lower potential for potentially damaging SO2 oxidation.
Iron- and copper-exchanged zeolite urea SCRs have been developed with approximately equal performance to that of vanadium-urea SCRs if the fraction of the NO2 is 20% to 50% of the total NOx. The two most common designs of SCR catalyst geometry used today are honeycomb and plate structures. The honeycomb form usually is an extruded ceramic applied homogeneously throughout the ceramic carrier or coated on the substrate. Like the various types of catalysts, their configuration also has advantages and disadvantages. Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, but plate configurations are much larger and more expensive. Honeycomb configurations are smaller than plate types, but have higher pressure drops and plug much more easily. A third type is a corrugated design, comprising only about 10% of the market in power plant applications.
The first large-scale SCR was installed by the IHI Corporation in 1978. Using modern selective catalytic reduction methods, it is now possible to remove 70 to 95% of the nitrogen oxides that are released into the environment. The amount that is removed depends on the specific SCR method used and the type of operation it is used with.
SCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many SCRs are given a finite life due to known amounts of contaminants in the untreated gas. The large majority of catalyst on the market is of porous construction. This porosity is what gives the catalyst the high surface area essential for reduction of NOx. However, the pores are easily plugged by a variety of compounds present in combustion/flue gas. Some examples of plugging contaminates are: fine particulate matter (PM), ammonia sulfur compounds, ammonium bisulfate (ABS), and silicon compounds. Many of these contaminants must be removed while the unit is on line, for example by soot blowers or particulate filters. The unit can also be cleaned during a turnaround or by raising the exhaust temperature. Of more concern to SCR performance is catalyst poisons, which can destroy the chemistry of the catalyst and render the SCR ineffective at NOx reduction, or cause unwanted oxidation of ammonia (forming more NOx). Some of these poisons include: halogens, alkaline metals, arsenic, phosphorus, antimony, chrome, and copper.
Most SCRs require tuning to properly perform. Part of the tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip, due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system. Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or the catalyst has degraded (see above). Temperature is one of the largest limitations of SCR. Gas turbines, cars, and diesel engines all have a period during a start-up where exhaust temperatures are too cool for NOx reduction to occur.
In coal-fired power stations, the same basic technology is employed for removal of NOx from the flue gas of boilers used in the power generation industry. In general, the SCR unit is located between the furnace economizer and the air heater, and the ammonia is injected into the catalyst chamber through an ammonia injection grid. As in other SCR applications, the temperature of operation is critical. Ammonia slip is also an issue with SCR technology used in power plants.
Other issues that must be considered in using SCR for NOx control in power plants are the formation of ammonium sulfate and ammonium bisulfate, due to the sulfur content of the fuel as well as the undesirable catalyst-caused formation of SO3 from the SO2 and O2 in the flue gas.
A further operational difficulty in coal-fired boilers is the binding of the catalyst by fly ash from the fuel combustion. This requires the usage of soot-blowers, sonic horns, and careful design of the ductwork and catalyst materials to avoid plugging by the fly ash. SCR catalysts have a typical operational lifetime of about 16,000-40,000 hours in coal-fired power plants, depending on the flue gas composition, and up to 80,000 hours in cleaner gas-fired power plants.
All the above processes are either energy intensive, or consume raw materials which cannot be easily regenerated.